Technical Field
The present invention relates to the technical field of lightweight high-strength structural materials, and in particular to a TiAl intermetallic compound single crystal material and a preparation method therefor.
Related Art
TiAl intermetallic compound is a new type of lightweight and high-temperature structural material, having a specific gravity that is less than 50% of that of a nickel-based high-temperature alloy, and having high specific strength, high specific stiffness, corrosion resistance, wear resistance, high temperature resistance, and high elastic modulus, as well as excellent oxidation resistance, creep resistance and good high-temperature strength and so on. The temperature during use can be up to 750-900° C., which is close to a Ni-based high-temperature alloy, but the density is only half of the high-temperature alloy. Therefore, the TiAl intermetallic compound is an ideal material for replacement of the Ni-based high temperature alloy, can be widely used in high-temperature components of car or aeroengines such as blades, turbine discs and exhaust valves. For example, the TiAl alloys are high-temperature materials for aerospace applications in a weight reduction unit of gram, especially the best candidate materials for engines. The last two stages of blades of a low pressure turbine in Boeing 787 aircraft are successfully developed by GE with the Ti-48Al-2Cr-2Nb(4822) alloy, to permit the aircraft to have a weight reduction of about 200 Kg. The high Nb TiAl alloy is obviously advantageous over ordinary TiAl alloys in terms of the high-temperature mechanical properties, creep resistance and oxidation resistance, and has an operating temperature increased by about 60-100° C., thus being a TiAl alloy having the most promising prospect in engineering applications.
However, due to the intrinsic brittleness of intermetallic compounds, the poor brittleness of the TiAl alloy at room temperature is a main factor that hinders its industrial application. Moreover, the working temperature of the 4822 alloy used at present is only 650° C., and the high-temperature performance needs to be further improved. Therefore, a large number of studies focus on regulating the microstructure of the TiAl alloy to improve the brittleness at room temperature and improve the working temperature. Due to the apparent anisotropy of the strength and plasticity of the PST crystal of TiAl alloy, a PST crystal with fully lamellar structure is fabricated with the Ti—Al alloy by directional solidification, in which the lamellar structure is oriented parallel to the growth direction of the crystal in the directional solidification, thereby improving the mechanical properties of the TiAl alloy.
The mechanical performance of a TiAl alloy with fully lamellar structure is closely related to its lamellar orientation. By studying the polysynthetic twinned crystal (PST) with a single orientation, it is found that the strength and plasticity are obviously anisotropic. Due to this anisotropy of the fully lamellar structure, it is more adaptable to the service conditions for blades in aircraft engines such as those requiring high temperatures and is amenable only to one-dimensional load. It is undoubtedly extremely advantageous if the TiAl alloy can be made by directional solidification into an engine blade with fully lamellar structure and the lamellar structure is oriented parallel to the axial direction of the blade (the direction of crystal growth in directional solidification). Yamaguchi et al. systematically studied the effect of the lamellar orientation of TiAl alloy on the mechanical properties. It was found that the combination of the yield strength and elongation was optimal, when the loading direction was parallel to the lamellar orientation. Therefore, to further improve the performance of the TiAl alloy in use, it is necessary to control the lamellar orientation of the final structure, so as to obtain a fully lamellar structure of a TiAl intermetallic compound single crystal having an orientation that is in agreement with the load direction.
At present, the methods for controlling the lamellar orientation of the TiAl alloy at home and abroad mainly include seeded method and non-seeded method to change the solidification path. A single crystal PST in which the lamellar orientation is completely parallel to the growth direction is obtained by Yamaguchi and Johnson et al through the seeded method of α-phase solidification using a Ti—Al—Si-based alloy as seed crystal and using necking and crystal selection. The difference between the compositions of the seed crystal and the master alloy usually leads to the uneven composition and performance of the alloy obtained after directional solidification. Moreover, the preparation process of the seed crystal is complex. Therefore, the seeded method has obvious shortcomings.
No fully lamellar TiAl single crystal structures with a lamellar orientation parallel to the growth direction are developed currently by using the non-seeded method at home and abroad. A fully lamellar single crystal structure with a lamellar orientation parallel to the growth direction is obtained from the Ti-46Al-5Nb alloy by Lin Junpin et al using “double directional solidification” at a low G/V condition. It is considered that an α phase with a single orientation parallel to the growth direction can be obtained by the peritectic reaction of the β phase with dendritic spacing in the process with a low G/V under appropriate conditions, without the β→α solid/solid phase transition to generate different phases of the alpha variants, thus accomplishing the control of the lamellar orientation. This method requires two times the same process of directional solidification, more than ordinary non-seeding method more than a solidification process, increasing the crucible material on the alloy pollution, the directional solidification of TiAl alloy industrialization. This method requires two identical directional solidification processes, which is one solidification process more than the ordinary non-seeded method, thus increasing the pollution of the alloy with the crucible material, and being adverse to the industrialization of the TiAl alloys obtained by directional solidification.
Previous studies on the control of lamellar orientation through non-seeded methods at home and abroad are to change the solidification path, which fails to control the lamellar orientation of a single crystal and fails to obtain a single crystal with a lamellar structure that is completely parallel to the growth direction. In order to solve this technical problem, the directional solid phase transition process of the TiAl alloy becomes key to control the lamellar orientation. It can be seen from the phase diagram that after solidification, the TiAl alloy with fully lamellar structure undergoes the solid phase transitions of β→α and α→α2+γ. When the primary phase is a β phase, the preferential growth direction is <001>, and the phase relation is {110}β//{0001}α//{111}γ[25]. 4 out of 12 variables for {110}β are parallel to the growth direction, 8 is inclined at 45° with respect to the growth direction[16,26], and after the solid phase transition, only ⅓ of the habit plane in the lamellar structure formed has an orientation that is parallel to the growth direction. It is clear that the final orientation of the lamellar structure of the TiAl alloy depends not only on the growth direction of the primary β phase but also on the subsequent solid phase transition process. Therefore, the β→α solid phase transition process is also key to control the lamellar orientation. So far, the studies on the control of the lamellar orientation of TiAl are focused on the solidification process; however, the solid phase transition process after solidification is ignored.
Therefore, it is necessary to control the solidification process so that the primary phase upon directional solidification is a β phase, and the nucleation and growth of a new phase and the directional phase boundary migration in the directional solid phase transition of the TiAl alloy also need to be controlled, such that only a lamellar orientation that is 0° with respect to the growth direction is retained during the directional solid phase transition, thereby accomplishing the control of the lamellar orientation of the TiAl alloy during the continuous directional liquid/solid-solid/solid phase transition.